f(k) is basically to represent a part of the function in an infinite series where we can use any value (any real with the exception of one value) in place of 'k' and still get a series equal 'e'. Just to clarify this does not mean some simple function where the 'k's cancel, each series will be unique for each value of 'k'.

This would be much easier if I could just post the identity but the moderators have made it very clear that this is against guidelines as 'no personal theories are allowed' even though this is mathematics and based on proofs.

Anyway the best I have seen is on wikipedia with this family of functions:

This would be much easier if I could just post the identity but the moderators have made it very clear that this is against guidelines as 'no personal theories are allowed' even though this is mathematics and based on proofs.

I vaguely remember you saying that mathematica or wolfram alpha could verify it. In that case it's not really a new personal theory, so I'll allow it.

I vaguely remember you saying that mathematica or wolfram alpha could verify it. In that case it's not really a new personal theory, so I'll allow it.

That was for a reciprocal gamma function, this one is not verified by mathematica but for summations up to 1000 terms has been perfect. If the moderators have no issue without a formal proof then let's take a closer look.

That was for a reciprocal gamma function, this one is not verified by mathematica but for summations up to 1000 terms has been perfect. If the moderators have no issue without a formal proof then let's take a closer look.

If you post your formula and if you ask us whether we saw this before or whether we have a reference, then I have no qualms about you posting it.

You can manipulate that expression like micromass did with the other one and see that it isn't anything new.

Micromass laid out a beautifully simple proof and it can easily be used to show the latter is also true ('k' and 'm') by replacing the '1' with an 'm' so this function is easy to prove as well.

An interesting thing about proofs, I had one of my Calculus Professors review another identity I wrote and he was eventually able to write a simple proof for it as well and he quickly declared it therefore must be known. The research Professors at my University took a different attitude towards the identity and have instructed me to "not waste my time with him".

I am new to this so don't know the difference between what is known or not. Can you perhaps show a reference to the identity? This general series can produce an infinite number of unique series expansions for 'e' so it would seem this should be commonly known but I have been unable to turn it up.

You're right, your identity isn't commonly known and it is probably even new. But that doesn't make it very useful. So finding a new identity is easy, but you should also really explain why people would be interested in such an identity. I don't think your identity doesn't offer anything new.

Generalize further you have
$$\left(C_1+C_2 x\dfrac{d}{dx}\right)\sum_{n=0}^\infty a_n x^n=\sum_{n=0}^\infty \left(C_1+C_2 x\dfrac{d}{dx}\right) a_n x^n=\sum_{n=0}^\infty \left(C_1+C_2 n\right) a_n x^n$$
or further still
$$p\left( x\dfrac{d}{dx}\right)\sum_{n=0}^\infty a_n x^n=\sum_{n=0}^\infty p\left( x\dfrac{d}{dx}\right) a_n x^n=\sum_{n=0}^\infty p\left( n\right) a_n x^n$$
for a polynomial p
which can be proved by using uniform convergence to justify term by term differentiation

You're right, your identity isn't commonly known and it is probably even new. But that doesn't make it very useful. So finding a new identity is easy,

Are you referring to something along the lines of what lurflurf posted?

I've hardly brought Calculus into series yet (outside of writing series for the Euler Mascheroni constant and 0th degree polygamma general solutions I haven't used it at all).

What other series do we have for creating an infinite number of unique series expansions for 'e'? I am up to a whopping '2' (including this one :P), math is so broad today it can be difficult to find your way around...

For usefulness I would say the formula is more useful the other way
That is I cannot think (though there are surely some) of an example where
$$\sum_{n=0}^\infty a_n x^n=\left[ p\left( x\dfrac{d}{dx}\right)\right]^{-1} \sum_{n=0}^\infty p\left( n\right) a_n x^n$$
would be useful, however
$$\sum_{n=0}^\infty p\left( n\right) a_n x^n=p\left( x\dfrac{d}{dx}\right)\sum_{n=0}^\infty a_n x^n$$
is a very useful way to find
$$\sum_{n=0}^\infty p\left( n\right) a_n x^n$$
when we know
$$\sum_{n=0}^\infty a_n x^n$$
These come up all the times in many areas such as probability where besides examples involving e^x we have many involving 1/(1-x) like
$$\sum_{n=0}^\infty n^2 x^n$$
and sums like
$$\sum_{n=0}^N n^3=\lim_{x\rightarrow 1} \sum_{n=0}^N n^3 x^n$$
That you can now easily calculate

For usefulness I would say the formula is more useful the other way
That is I cannot think (though there are surely some) of an example where
$$\sum_{n=0}^\infty a_n x^n=\left[ p\left( x\dfrac{d}{dx}\right)\right]^{-1} \sum_{n=0}^\infty p\left( n\right) a_n x^n$$
would be useful, however
$$\sum_{n=0}^\infty p\left( n\right) a_n x^n=p\left( x\dfrac{d}{dx}\right)\sum_{n=0}^\infty a_n x^n$$
is a very useful way to find
$$\sum_{n=0}^\infty p\left( n\right) a_n x^n$$
when we know
$$\sum_{n=0}^\infty a_n x^n$$
These come up all the times in many areas such as probability where besides examples involving e^x we have many involving 1/(1-x) like
$$\sum_{n=0}^\infty n^2 x^n$$
and sums like
$$\sum_{n=0}^N n^3=\lim_{x\rightarrow 1} \sum_{n=0}^N n^3 x^n$$
That you can now easily calculate

I bet there is a lot of good information in your post however I am having trouble figuring out what you are trying to show. After looking at it for awhile yesterday I eventually threw in the towel and went to the advanced math tutoring lab and spoke with one of our grad students (specializes in algebraic number theory) but he wasn't sure what you were getting at either.

Unfortunately if the graduate Mathematics major is confused then this lowly Engineering undergrad doesn't stand a chance. I bet your post is chock full of useful information so I will attempt the tutoring lab again today, if all else fails then I will go knocking on some doors.

So in the interim, are you aware of any other series summations 'general solutions' capable of producing an infinite number of unique series summations for 'e'?

Could you explain the parts that are confusing?
I have questions too.
Back in #3 do you know how the numbers in the family can be calculated?
Do you see how
$$\sum_{n=0} p(n)/n!=C \, e$$
For some constant c and how to find C?
How much calculus do you know?
Do you understand Taylor's series?
Do you understand the generalization of your formula I GAVE IN #15 hand how it gives the above formula and a way to find C?
The links give general methods of expanding functions in series, they are not given in elementary calculus because they are complicated.
Another series is
$$e=\frac{1}{1+\mathrm{erf}(1)}\sum_{n=2}^\infty \frac{1}{\Gamma(n/2)}$$
we can give series for e all day, but to what end?
The notation $$\left(x \, \dfrac{d}{dx}\right) ^n\mathrm{f}(x)$$
means apply n times ie
$$\left( x \, \dfrac{d}{dx}\right) ^4\mathrm{f}(x)=\left( x \, \dfrac{d}{dx}\left(x \, \dfrac{d}{dx}\left(x \, \dfrac{d}{dx}\left(x \, \dfrac{d}{dx}\mathrm{f}(x)\right)\right)\right)\right)$$

Could you explain the parts that are confusing?
I have questions too.
Back in #3 do you know how the numbers in the family can be calculated?
Do you see how
$$\sum_{n=0} p(n)/n!=C \, e$$
For some constant c and how to find C?

Okay, so you are saying if we have some polynomial of 'n' in the numerator it will be equal to some value 'C' and you want to know if I can calculate 'C'? If you mean as in using some 'formula' for calculating it, no, but if given the polynomial I can figure it out.

'e' is a fundamental mathematical constant, anything that gives more insight or variability to it I would think is important. For example, we can use the identity I posted and with some manipulation can produce the same for ex.